Depending on the particular application, especially what type of vehicle, a crash or collision with another vehicle or an obstacle, the direction and magnitude of a shock, the efforts to be made to sufficiently protect a passenger against injuries are different, but sensing and evaluating the crash has to be done in the range of milliseconds in order to have enough time to inflate or to tension the above mentioned restraint system. An igniting device must be activated in order to ignite a fuel charge or propellant in the gas generator when triggered.
It is known from prior art, that the activation should be performed only when a distinct acceleration (or deceleration in the same order but negative) is reached and was sensed by means of a moveable spring-and-weight sensing and/or triggering system. These have primarily been referred to as the mechanical sensor. Such sensor systems are described and shown in U.S. Pat. Nos. 4,167,276, 4,711,466 and 4,552,380.
Because electronic sensor systems could sense and evaluate more eventualities regarding vehicle type and crash events they have become widespread in practice, than mechanical sensors. However to avoid influences of electromagnetic waves or the possible influence of High Frequencies (HF), from radio stations and the like, in electronic circuits in vehicles during operation, most of the known electronic sensors for restraint systems have--for safety reasons also a special safing sensor in form of a mechanico-electrical switch, normally open, and closed only when a substantial velocity change occurs.
In addition, the prior art devices of restraint systems have a solid fuel-filled gas generator to inflate an airbag or to tension a belt tensioner and an igniter/primer is structurally integrated with the gas generator housing.
In practice a gas generator in a ready-to-operate state poses a potential hazard with regard to the installation of the generator into a vehicle in two respects:
a) electrically: all electrical connections must be periodically checked to ensure proper functioning, and PA1 b) chemically: car manufacturers desire guaranties as to safety against any form of leaks in the gas generator and also against self ignition of the gas generator fuel within the normal lifetime of such vehicle. Furthermore since in known systems the igniter/primer and the solid fuel (main charge) are integrated in the gas generator housing, the disposal of such units which must be disposed off, even after years of non-use, is a burden to environment.